Eurotunnel has warned customers hoping to beat the government's latest travel quarantine deadline not to just turn up at its terminal on Friday.
People coming to the UK from France and the Netherlands will be forced to quarantine for 14 days if they arrive back after 04:00 on Saturday.
The firm warned hopeful passengers: "Do not turn up unless you have a booking. You won't get on a crossing."
"This being a peak August weekend, we are already full up with bookings."
John Keefe, director of public affairs at Getlink, which operates the Channel Tunnel, told the BBC: "The service is already very busy this weekend and there is no additional capacity."
Customers will be unable to board alternative shuttles without a valid booking, he added.
"To avoid long queues and severe disruption, we strongly advise against turning up at the terminal outside the allocated time," Eurotunnel said.
If people with a booking want to change it, they need to do so online, it said.
"Customers currently in France, and hoping to return early, must amend their tickets online at www.eurotunnel.com, before travelling to the French terminal," the company said.
But there have been long queues at the company's website since the news was announced on Thursday.
It is estimated there are about half a million British people holidaying in France at the moment.
Eurotunnel said on a normal Friday in August, it would transport 10-11,000 cars from France to the UK.
No extra crossings
Airlines and ferries are also unlikely to provide a route home for people rushing to beat the deadline.
Brittany Ferries told the BBC: "There won't be any more crossings."
'We feel devastated'
Just two days into a 12-day road trip and hopes of seeing family in England for the first time in months, Tyler Peters and his girlfriend Julia have been left feeling bewildered in Amsterdam.
The couple had set out from their Berlin home on Wednesday and after a brief stop in the Dutch capital, were booked on a late night ferry from the Hook of Holland to Harwich on Friday night.
But their ferry was due to arrive in England at 6.00am, two hours after the 4.00am quarantine deadline set by the British government.
"It meant we wouldn't even be able to see my mother in Cambridgeshire, who has been shielding for months, because we'd have to self-isolate for two weeks," said Tyler.
"We looked into getting an earlier ferry but we couldn't afford the extra €350, which means we've had to scrap our plans and lose the €250 we'd already paid for non-refundable ferry tickets."
It was the couple's first break since lockdown and they'd planned to drive across Europe as they felt it was safer than flying.
After visiting Tyler's family, they'd intended driving back through France, Belgium and Austria before returning to Berlin.
"We had just wanted to do as much as possible in the short period of time we had available, and within our limited funds," Tyler said.
Now all those plans are in disarray.
"Our plans had to change within moments and we feel devastated," Tyler said.
"We're trying to figure out what to do now but it's not easy finding out any helpful information to advise on how travellers like us should behave.
"We're probably just going to get out of the Netherlands as soon as possible and just go back to Germany."
EasyJet said its schedule would operate as planned. "We plan to operate our full schedule in the coming days," it said.
"Customers who no longer wish to travel can transfer their flights without a change fee or receive a voucher for the value of the booking."
British Airways said of its flights to France and Netherlands: "We are currently operating a limited schedule to these countries.
"As always, if a customer's flight is cancelled, they are entitled to a voucher or a full refund."
Jet2, meanwhile, said it would continue to operate flights to Bergerac and La Rochelle in France.
However it will not fly to any other French destinations until at least October.
British Airways said it was running a limited schedule to France, the Netherlands, Monaco, Malta, Turks and Caicos and Aruba, which are all on the quarantine list. It said customers would be able to claim a refund if their flight was cancelled.
Companies linked to the travel industry saw their share prices drop on Friday.
EasyJet's share price dropped 6.61% to 570.46p, while British Airways' parent, International Airlines Group, saw its stock price fall 6.4% to 191.25p.
Ryanair's price fell by 4.7%.
The price of holiday firm Tui's shares dropped 5.3% to 326.2p each.